Black holes for everyone

Theorists investigate the probability of black hole production at the Large …

The Large Hadron Collider (LHC), due to come online sometime this year, will be capable of the highest energy collisions ever attempted by mankind. One of the questions raised during its design was the probability of creating black holes and subsequently destroying the Earth. It has since been decided that this is unlikely.

An interesting article has appeared in the European Journal of Physics, (free copy here) which essentially looks at what would have to be true to make black holes with the LHC. The amount of energy required to create a black hole depends on the strength of gravity. At macroscopic scales, this is well-known from Newton's law and its relativistic equivalent. Scaling from these laws tells us what the strength of gravity will be when we start knocking protons together—a boat load. If you visualize gravity as a bunch of lines expanding radially from a central mass, then this becomes obvious, the number of lines don't change, but they do get closer together—gravity sucks harder when you get closer to the central mass. However, if there are more than three spatial dimensions, the lines have all these other dimensions to spread out in, effectively weakening gravity. We know that the macroscopic world has only three dimensions; however, many quantum gravity theories posit extra dimensions on a small scale, which would reduce the amount of energy required to create a black hole.

The situation gets more complex though because the collisional energy of the LHC is also associated with a length scale, which is something less than an attometer (10-18m). To make use of these extra dimensions, they must be bigger than this scale. Thus, we have two conditions; there must exist extra dimensions, and the dimensions must be at least 0.01am in length. If this happens to be true, the LHC may be good at producing very small black holes. If Hawking is correct, these will almost instantly decay in a spray of quarks and other particles, which are readily detectable (though it remains an open question if their appearance will be linked to black hole formation). On the other hand, Hawking may be wrong and these black holes could be stable, forming a kind of dark matter. In this case, they will sink to the center of the Earth and begin devouring us all (at a rate of about one particle every few years.

Even if micro-black holes are stable, you should not be alarmed. The LHC is not the highest energy thing around as we are constantly bombarded by cosmic rays, some of which are more energetic than anything the LHC will produce. Taking the bombardment rates observed today, we can expect there to be about 0.5kg of black holes sitting at the center of the Earth today.

The final thing I will say is that the linked paper is not really a research paper, but rather it is aimed at students. Even if you don't follow the maths (and it really isn't necessary to), it is worth reading.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.